Optical sensor for surface inspection and metrology

ABSTRACT

An interferometer and an imager may include a tunable light source, a beam splitter, a digital imager, and a processor system. The tunable light source may be configured to emit a beam. The beam splitter may be configured to direct the beam toward a sample with a floor surface and a raised surface feature. The digital imager may be configured to receive a reflected beam and to generate an image based on the reflected beam. The reflected beam may be a coherent addition of a first reflection of the beam off a reference plate and a second reflection of the beam off the raised surface feature and third reflection of the beam off the floor surface. The processor system may be coupled to the digital imager and may be configured to determine a distance between the reference surface and the feature surface based on the image. A second digital imager may also be configured to receive a reflected beam and scattered beam to generate a two-dimensional grayscale image of the surface based on these beams and may also be configured to receive fluorescent light generated by the incident light to generate a two-dimensional gray scale an image of the surface based on fluorescent emission.

FIELD OF THE INVENTION

The embodiments discussed in this disclosure are related to an imager and imaging interferometer.

BACKGROUND OF THE INVENTION

An interferometer utilizes superimposed waves, such as visible light or electromagnetic waves from other spectral regions, to extract information about the state of the superimposed waves. The superimposition of two or more waves with the same frequency may combine and add coherently. The resulting wave from the combination of the two or more waves may be determined by the phase difference between them. For example, waves that are in-phase may undergo constructive interference while waves that are out-of-phase may undergo destructive interference. The information extracted from the coherently added waves may be used to determine information about a structure that interacts with the waves. For example, interferometers may be used for measurement of small displacements, refractive index changes, and surface topography. In imaging interferometer, interferometric measurements over a finite area of the surface, instead of a point on the surface, is achieved by using imaging sensors. An imager utilizes reflected and scattered waves, such as visible light or electromagnetic waves from other spectral regions, to extract information about the state, location, and topology of surface defects.

The subject matter claimed in this disclosure is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology area where some embodiments described in this disclosure may be practiced.

SUMMARY OF THE INVENTION

According to an aspect of one or more embodiments, an interferometer may include tunable light sources, splitters, digital imagers, and processor systems. The tunable light source may be configured to emit a light beam. The splitters may be configured to direct the beam toward a sample with a reference plate, a raised surface feature and a feature floor surface. According to an embodiment, a first digital imager may be configured to receive a reflected beam to generate an interference pattern and to generate an image based on the reflected beams. The reflected beam may be a coherent addition of a first reflection of the beam off the surface of the reference plate, a second reflection of the beam off the raised surface feature and a third reflection off the floor of the sample or substrate on which features are located. The processor system may be coupled to the digital imager and may be configured to determine a distance between the reference surface and the raised surface features based on the generated image or images.

According to an aspect of one or more embodiments, methods of determining the thickness of a sample are also disclosed. The methods may include emitting a light beam and directing the light beam toward a sample with a floor surface and a feature surface. The methods may also include generating an image based on a reflected light beam. The reflected light beam may be a coherent addition of a first reflection of the light beam off the reference plate, a second reflection of the light beam off of the top surface of the sample and a third reflection of the light beam off of the bottom surface of the sample. The method may also include determining a thickness between the top surface and the bottom surface based on the reflections.

A second digital imager may be configured to receive reflected and scattered beams to generate an image of protruding features and the sample floor on which the features are positioned. The processor system may be coupled to the second digital imager and may be configured to build a two-dimensional image of the raised surface features and the floor of the sample.

One embodiment is a method, comprising emitting a first light beam of a first wavelength at a first time; directing the first light beam toward a reference plate and a first sample facing the reference plate, the first sample defining a floor surface and a raised surface feature rising above the floor surface; generating a first image based on a first reflected light beam, the first reflected light beam being a coherent addition of a first reflection of the first light beam off of the reference plate, a second reflection of the first light beam off of the raised surface feature and a third reflection of the first light beam off of the floor surface, and based on the generated first image, determining a distance between the surface of the reference plate and the raised surface feature and determining a distance between the surface of the reference plate and the floor surface.

According to further embodiments, the method may further comprise emitting a second light beam of a second wavelength at a second time different from the first time; directing the second light beam toward the first sample; generating a second image based on a reflected second light beam, the reflected second light beam being a coherent addition of a first reflection of the second light beam off of the reference plate, a second reflection of the second light beam off of the raised surface feature and a third reflection of the second light beam off of the floor surface, and based on the generated first and second images, determining the distance between the surface of the reference plate and the raised surface feature, and a distance between the reference plate and the floor surface.

The method may further comprise selecting a wavelength difference between the first wavelength and the second wavelength based on a distance between the floor surface and the raised surface feature; and selecting a first bandwidth of the first light beam and a second bandwidth of the second light beam based on a distance between the surface of the reference plate and different raised surface features on the first sample. According to one embodiment, determining the distance between the reference plate and the raised surface feature based on the first image and the second image may include constructing a fringe pattern based on first intensity values of pixels from the first image of and second intensity values from pixels of the second image; and performing a frequency domain transform on the constructed fringe pattern. For instance, the frequency domain transform may be a Fast Fourier Transform, Discrete Fourier Transform, a Discrete Cosine Transform or a Hilbert Transform.

In one embodiment, determining the distance between the reference plate and the raised surface feature based on the first image may comprise comparing an intensity of a pixel of the image to a plurality of model-based pixel intensities that correspond with respective different distances; and selecting one of the plurality of model-based pixel intensities that is closest to the intensity of the pixel, wherein the determined distance is the distance corresponding to the selected one of the plurality of model-based pixel intensities.

According to one embodiment, the first sample is part of a semiconductor formed on a wafer, and the method may further comprise emitting a second light beam; directing the second light beam toward a second sample of the semiconductor, the second sample being unilluminated by the first light beam and located on a different part of the semiconductor than the first sample; generating a second image based on a reflected second light beam, the reflected second light beam being a coherent addition of a first reflection of the second light beam off of the reference plate, of a second reflection of the second light beam off of a raised surface feature on the second sample and a third reflection of the second light beam off of the floor surface of the second sample; and based on the second image, determining second distances between the reference plate and the raised surface feature of the second sample, and between the reference plate and a floor surface of the second sample.

In one embodiment, an auto-correlation interferometer function may be carried out on at least the first reflected light beam.

According to some embodiments, directing the first light beam may comprise directing the first light beam toward the reference plate and the first sample at an angle normal or substantially normal to the reference plate. The method may also comprise a second light beam; directing the second light beam toward the reference plate and the first sample at angles away from normal to the floor surface of the sample; generating a second image based on a second reflected light beam, the second reflected light beam being a coherent addition of a second reflection/scatter of the second light beam off of the reference plate, a second reflection/scatter of the second light beam off of the raised surface feature and a third reflection/scatter of the second light beam off of the floor surface; and based on the generated second image, determining the distance between the reference plate and the raised surface feature and determining the distance between the reference plate and the floor surface.

Another embodiment is an optical system configured to measure raised surface features on a surface of a sample. The optical system may comprise a first digital imager and a second digital imager; a processor system coupled to the first and to the second digital imagers; a tunable light source configured to emit a first light beam and a second light beam; a first lens system (comprising one or more lenses) configured to focus incident light onto the sample surface and to focus reflected light back on the first and second digital imagers; a reference plate disposed above and facing the sample surface; an off-axis ring illuminator and configured to receive and direct the second light beam toward the reference plate at angles other than normal to the sample surface; a first beam splitter configured to direct the first light beam through the first lens system toward the reference plate in a direction normal to the sample surface and to transmit a first reflected light beam of the first light beam reflected by the reference plate and a second reflected light beam of the second light beam reflected by the a raised surface feature on the sample; a second beam splitter configured to transmit at least a portion of the first reflected light beam toward the first digital imager and to reflect at least a portion of the second reflected light beam toward the second digital imager; wherein the first digital imager is configured to generate a first digital image based at least on the first reflected light beam and wherein the second digital imager is configured to generate a second digital image based at least on the second reflected light beam, and wherein the processor system is configured to determine a distance between the reference plate and the raised surface feature of the sample based at least on the first and second digital images

The first reflected light may comprise a coherent addition of reflections of the first light beam on the reference plate, on the raised surface feature and on the sample surface. Similarly, the second reflected light may comprise a coherent addition of reflections of the second light beam on the reference plate, on the raised surface feature and on the sample surface.

In one embodiment, the off-axis ring illuminator is disposed co-axially with the first lens system.

The processor system may be further configured to generate an image of the sample surface for defect inspection and to generate a fluorescence image of the sample for residue-defect inspection. According to some embodiments, the second beam splitter may include a dichroic mirror configured to primarily transmit a first pre-determined band of wavelengths and to primarily reflect a second pre-determined band of wavelengths, wherein the first light beam has a wavelength within the first pre-determined band of wavelengths and wherein the second light beam has a wavelength within the second pre-determined band of wavelengths.

According to an embodiment, the first digital imager and the second digital imager may be configured to generate digital images based on sample fluorescence emission, generated via excitation of the sample surface and of the raised surface feature by the first and second light beams.

The first and/or the second digital imagers may comprise, for example, a two-dimensional array of CCD or CMOS pixels or a one-dimensional line array of CCD or CMOS pixels having pixel readout rate ranging from tens of Hz to hundreds of kHz, such as from 10 Hz to 300 KHz.

The reference plate positioned above the sample surface may have an optical thickness configured to provide a near common path Mirau interferometer functionality.

The optical system may be further configured to optically scan the sample surface using a translation system whose travel speed across a field of view of the optical system determines a sampling pixel size of the first and second images. The translation system may be configured to move the sample across the field of view of the optical system and/or to move the optical system across the sample.

According to some embodiments, the first light beam has a first wavelength and is emitted at a first time, and the second light beam has a second wavelength that is different form the first wavelength and is emitted at a second time that is different than the first time. The processor system may be configured to determine the distance between the reference plate and the raised surface feature based on the first digital image and the second digital image. The tunable light source may be configured to emit a plurality of light beams, the plurality of light beams including the first light beam and the second light beam, each of the plurality of light beams having a different wavelength, a number of the plurality of light beams being selected based on the distance between the reference plate surface and the raised surface feature. The determined distance between the reference plate surface and the raised surface feature may then be based on a plurality of first and second images generated by the first and second digital imagers based on the plurality of light beams. The tunable light source may comprise a broadband light source configured to emit the first light beam at a first time and the second light beam at a second time; and a tunable filter that is configured to filter the first light beam to have a first wavelength and to filter the second light beam to have a second wavelength.

According to one embodiment, the distance between a reference surface on the sample or the reference plate and the raised surface feature may be determined based on a first intensity value of a first pixel location in the first digital image and a second intensity value of a corresponding first pixel location in the second digital image.

The first lens system may be positioned between the sample and the first beam splitter, and the optical system may further comprise a second lens system positioned between the first and the second beam splitters; and an adjustable system aperture positioned one in an aperture plane of the first lens system, or between the first lens system and the second lens system. The size of an aperture of the adjustable system aperture may be configured to be adjustable based on a lateral resolution and/or a field of view of the optical system.

In one embodiment, the first beam splitter may be movably controllable to redirect the first light beam away from the raised surface feature toward an other part of the sample, such that the raised surface feature is not illuminated (unilluminated) by the redirected first light beam and the processor system may be configured to determine a distance between the reference plate and the other part of the sample.

According to some embodiments, the second digital imager may be configured to receive at least a portion of the first light beam that includes a reflected light beam from normal incidence illumination and at least a portion of the second light beam that includes scattered light beam from off-axis illumination to generate at least one third digital image based on the reflected and scattered light beams and the processor system may be further configured to generate a two-dimensional digital image from the at least one third digital image.

In one embodiment, the sample may be moved under the imaging system by a x-y translation system comprising a moving stage and wherein the second digital imager may be configured to receive at least a portion of the first light beam that includes a reflected light beam from normal incidence and at least a portion of the second light beam that includes scattered light beam from off-axis illumination to generate third digital image(s) based on the reflected and scattered light beams. The processor system may be further configured to generate a two-dimensional digital image of an entire surface of the sample from the third digital image(s).

According to some embodiments, the tunable light source may be configured to emit a single wavelength of light beam toward the sample and to enable repeated multiple single wavelength inspection of the sample in both a bright field mode and in a dark field mode. The single wavelength of light beam incident on the sample surface may have a wavelength that causes the sample surface to fluoresce, and the second digital imager may be configured to receive a fluorescence emission from the sample surface via a first dichroic mirror as the sample is moved under the imaging system by translation system comprising a moving stage; and the second processor system may be configured to generate a two-dimensional digital image based on the received fluorescence emission.

The optical system may further comprise an auto-correlation interferometer comprising a first movable mirror and a second movable mirror both disposed along an optical path of the second reflected light beam and aligned with the second digital imager

The object and advantages of the embodiments will be realized and achieved at least by the elements, features, and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1A illustrates an example interferometer system according to an embodiment;

FIG. 1B illustrates multiple interferometer sub-systems according to an embodiment;

FIG. 2 illustrates another example interferometer system according to an embodiment;

FIG. 3 illustrates an example interferometer that includes auto correlation interferometry according to an embodiment;

FIG. 4 illustrates an example of beam reflection off an example semiconductor device according to an embodiment; and

FIG. 5 illustrates an example of beam reflection off another example semiconductor device according to an embodiment.

DESCRIPTION OF EMBODIMENTS

According to at least one embodiment, an interferometer system may include a tunable light source, a beam direction unit, an imaging lens assembly, a digital imager, and a processor system. The interferometer may be configured to determine a distance between a reference surface, a raised surface feature and floor surface of a sample. The sample may be a portion of a surface of a semiconductor device constructed or formed on a wafer. In some embodiments, the reference surface may be a top surface of the wafer and the feature surface may be a top surface of the semiconductor device formed on the wafer. In other embodiments, the reference surface may be a transparent surface not part of the wafer and the raised surface feature may be a top surface or floor surface of the semiconductor device built on the wafer.

In some embodiments, the tunable light source may be configured to emit a light beam with a first wavelength at a first time. The beam splitter may be configured to direct the beam toward the sample. The beam may reflect off the sample. In some embodiments, the beam may reflect off a reference surface to generate a first reflected beam. The beam may also reflect off the raised surface feature to generate a second reflected beam and a third reflected beam off of the reference surface, such as a reference plate. The first, second and third reflected beams may add together coherently to form a reflected imaging beam that is received by the digital imager. The digital imager may be configured to generate a digital image based on an intensity of the reflected imaging beam.

The tunable light source may be configured to emit multiple other light beams, each at a different time. Each of the multiple other light beams may have a different wavelength. A digital image may be generated by the digital imager for each of the multiple other light beams in a similar manner as the digital image was generated for the light beam with the first wavelength.

The processor system may be configured to receive and compare the digital images from the digital imager or imagers. Based on the comparison between intensity values at the same pixel locations in the digital images, the processor system may be configured to determine a distance between the reference surface or reference plate and different raised surface features and floor surface on the sample.

In some embodiments, the sample may be a single location. Alternately or additionally, the sample may correspond to an area of the semiconductor. In these and other embodiments, the processor system may be configured to determine a topology of the sample over the area of the semiconductor based on the digital images. The topology may represent the distance between the reference plate and the floor of the sample and the distance between the reference plate and the raised surface features over the area of the semiconductor substrate.

In some embodiments, the interferometer may include one or more lenses or lens systems and an adjustable system aperture in the imaging path (i.e., the path of the reflected light beams). The lens or lenses may be configured to focus the imaging beam on the digital imagers. The adjustable system aperture may be configured to adjust the spatial resolution of the imaging system. In these and other embodiments, the field of view of the digital imagers may correspond to the area of the semiconductor substrate for which the distances between the reference plate and the raised surface features over the area of the semiconductor substrate are to be determined.

In some embodiments, a system may include multiple interferometer systems. In these and other embodiments, each of the systems may determine a distance between a reference surface on the sample (or a separate reference plate) and a feature surface of the semiconductor for different portions of the semiconductor at the or nearly the same time. In this manner, a topology of an entire semiconductor may be more quickly determined than when using a single interferometer.

In some embodiment, the interferometric imaging system may be configured to include a separate imaging channel that incorporates a second digital imager configured to generate a digital image based on an intensity of the return beam reflected and scattered from the sample surface.

Embodiments of the present disclosure will be explained with reference to the accompanying drawings.

FIG. 1A illustrates an exemplary system 100 a (the “system 100 a”), arranged in accordance with at least some embodiments described in this disclosure. The system 100 a may be configured to image a raised surface feature 113 a on sample 112 that is part of, for example, a semiconductor device 130. To generate the sample surface image, the system 100 a may include a tunable light source 102, a beam splitter 104 a for on-axis beam 118 a, an off-axis ring illuminator 108 for off-axis beams 118 b, digital imagers 114 and 116, and a processor system 128.

The system 100 a may also be configured, using light beams, to determine a distance between the raised surface feature 113 a and the floor surface 113 b, with respect to a reference plate 110, of sample 112 that is part of a semiconductor device 130. According to embodiments, the reference plate 110 may be coated for partial reflectance and partial transmittance and may be disposed facing and in close proximity (e.g., 100 microns away to a few millimeters away) from the sample 112. The (in one embodiment, static) reference plate 110 may be configured to function as a reference mirror of a Mirau interferometer. The reference plate 110, in this manner, may be positioned above the surface of the sample 112, which may be moved under the optical system 100 a by an x-y translation system including a moving stage.

The system 100 a may be deployed in any suitable application where a distance is to be measured. For example, in some embodiments, the raised surface feature 113 a may be top surface feature of a semiconductor device 130 and the floor surface 113 b may be a top or bottom or floor surface of a silicon substrate wafer that forms a substrate of the semiconductor device 130. In these and other embodiments, the semiconductor device 130 may be any circuit, chip, or device that is fabricated on a silicon wafer. The semiconductor device 130 may include multiple layers of the same or different materials between the raised surface feature 113 a and the floor surface 113 b. Alternately or additionally, the raised surface feature 113 a may be a micro electromechanical systems (MEMS) structure and the floor surface 113 b may be a surface on which the MEMS structure is built.

Alternately or additionally, the raised surface feature 113 a may be any type of interconnect feature used in 3D packaging and the floor surface 113 b may be the corresponding surface from which the interconnect features protrude. An example of a protruding feature and a reference surface is described with respect to FIG. 4. Alternately or additionally, the raised surface feature 113 a may be an embedded surface within a semiconductor device or some other device and the reference surface 113 b may be a top surface. An example of an embedded surface is described with respect to FIG. 5. Although FIGS. 1, 2, 4 and 5 illustrate certain feature surface configurations, the principles and operation of the systems described in FIGS. 1, 2, 4 and 5 may be applied to any feature surface configuration.

Alternately or additionally, the raised surface feature 113 a may be any type of interconnect feature used in 3D packaging protruding from surface 113 b and the reference surface may be determined relative to the reference plate 110. Indeed, distances to 113 a and 113 b may be measured with respect to the reference plate 110. An example of a protruding feature and a reference surface is described with respect to FIG. 4. Alternately or additionally, the raised surface feature 113 a may be an embedded surface within a semiconductor device or some other device and the reference surface may be a top surface. In this manner, the phrase “raised surface feature”, as used herein, may refer to structures rising above or disposed below the floor or some other reference surface of the sample. An example of an embedded surface is described with respect to FIG. 4. Although FIGS. 1, 2, 4 and 5 illustrates certain feature surface configurations, the principles and operation of the systems described in FIGS. 1, 2, 4 and 5 may be applied to any feature surface configuration.

The tunable light source 102 may be configured to generate and to emit a light beam 118. In some embodiments, the tunable light source 102 may be a broadband light source that is tunable to multiple different wavelengths. For example, the tunable light source 102 may be tuned in a stepwise manner over a range of frequencies. In some embodiments, the tunable light source 102 may have a bandwidth that is between 300 nanometers (nm) and 1000 nm, between 1000 nm and 2000 nm, or some other range of wavelengths. For example, the tunable light source 102 may have a wavelength bandwidth that is between 650 nm and 950 nm. In some embodiments, the tuning step of the tunable light source 102 may be more or less than 1 nm. The tunable light source 102 may provide the light beam 118 a at a first wavelength to the first beam splitter 104 a.

The first beam splitter 104 a may be optically coupled to the tunable light source 102, the sample 112, and the digital imager 116. The first beam splitter 104 a may be configured to receive the light beam 118 a and to direct or reflect the light beam 118 a towards the sample 112. After being directed by the first beam splitter 104 a, the light beam 118 a may strike the reference plate 110 and raised surface feature 113 a of the sample 112. Striking the reference plate 110 may generate a first light beam reflection 121 and striking the raised surface feature 113 a of the sample 112 may generate a second light beam reflection 122 along the same path as the first light beam reflection 121. Alternately or additionally, a portion of the light beam 118 a may traverse through the sample 112 to the surface 113 b and strike the surface 113 b. Striking the surface 113 b may generate a third light beam reflection 123.

The first light beam reflection 121 and the second beam reflection 122 may be directed back toward the first beam splitter 104 a. The third light beam reflection 123 may also be directed back toward the first beam splitter 104 a. In these and other embodiments, the first and second light beam reflections 121, 122 and the third light beam reflection 123 may coherently add to one another to form a reflected light beam 119 a. The tunable light source 102 may also be configured to generate a second light beam 118 b, which may be directed onto the off-axis ring illuminator 108. The off-axis ring illuminator 108 may be configured to direct the second light beam 118 b toward the reference plate 110 and the sample 112 at angles other than normal to the surface of the reference plate 110 and of the floor of the sample 112. Off-axis incident light may then be reflected/scattered by, for example, raised surface features on the sample as reflected beam 119 b, as shown in FIG. 1.

The first beam splitter 104 a may be configured to receive the reflected light beam 119 a and the reflected off axis beam 119 b and pass the reflected light beam 118 a and the towards the digital imager 116 over interferometer channel 122 a. In one embodiment, the reflected light beam 118 a and the reflected off-axis beam 118 b may hit the first beam splitter 104 a, which then transmits a portion thereof though to the second beam splitter 104 b. The second beam splitter 104 b may be configured to reflects at least a portion of the reflected/scattered off-axis light beam 119 b over an imaging channel 122 b to the digital imager 114 and to transmit at least a portion of the reflected light beam 119 b over an interferometer channel 122 a to the digital imager 116. In the case in which the tunable light source 102 does not generate the second light beam 118 b and the off-axis ring illuminator 108 is not present, the second beam splitter 104 b may direct a portion of the reflected light beam 119 a toward the digital imager 114 and may direct a portion of the reflected light beam 119 a toward the digital imager 116. The second beam splitter 104 b may be configured for 50% transmittance (toward digital imager 116) and 50% reflectivity (toward digital imager 114), or some other transmittance/reflectance ratio.

When the tunable light source 102 does generate the second light beam 118 b (of a different wavelength/wavelengths than the first light beam 118 a) for darkfield inspection and the off-axis ring illuminator 108 is present, the beam splitter 104 b may include a dichroic mirror configured to primarily direct reflections from the first light beam 118 (having a first wavelength or band of wavelengths) to the digital imager 116 for interferometry and to primarily direct the reflections from the off-axis illumination light beam 118 b (having a second, different wavelength or band of wavelengths) to the digital imager 114 for darkfield inspection. The digital imager 114 may be configured to receive primarily the off-axis reflected light beam 119 b and to generate an image 124 based on an intensity of the reflected and/or scattered reflected off-axis light beam 119 b. Similarly, the digital imager 116 may be configured to primarily receive the reflected light beam 119 a and to generate an image 126 based on an intensity of the reflected light beam 119 a. In some embodiments, the digital imagers 114 and 116 may be CMOS or CCD type imagers or other types of 1D- or 2D-array detectors. In these and other embodiments, the first and second digital imagers 114, 116 may include multiple pixel elements. The pixel elements may be configured such that, when illuminated, each provides information about the intensity of the illumination that is striking the pixel element. The digital imagers 114, 116 may compile the information from the pixels to form the images 124 and 126. The images 124 and 126 may thus include the intensity information for each of the pixels. These images 124, 126, when they include intensity information for each pixel, may be referred to as a grayscale digital images, with the constituent grey levels thereof corresponding to the different levels of intensity of the illumination. As shown, the digital imagers 114 and 116 may provide the images 124 and 126 to the processor system 128.

The processor system 128 may be electrically coupled to the digital imagers 114 and 116. In these and other embodiments, the processor system 128 may receive the images 124 and 126. Based on these images 124, 126, the processor system 128 may be configured to determine a distance between the raised surface feature 113 a and the surface or floor 113 b of the sample 112 and to produce a grayscale inspection image of the raised features on the surface of the sample 112. For example, the height of the raised surface features above the floor surface of the sample may be characterized by determining the distance between the reference plate 110 and the raised surface feature and the distance between the reference plate 110 and the floor surface of the semiconductor. Subtracting the former from the latter may yield the height of the raised surface feature(s) above the floor surface of the sample.

In some embodiments, the tunable light source 102 may be configured to generate the light beam 118 a as a point light source with a small diameter beam. In these and other embodiments, the area of the sample 112 may be small and restricted to a particular location on the semiconductor device 130. In these and other embodiments, the distance between the raised surface feature 113 a and the floor surface 113 b may be determined for the particular location. Alternately or additionally, the tunable light source 102 may be configured to generate a wider light beam 118 a to illuminate a larger field of view. In these and other embodiments, the area of the sample 112 being imaged may be larger. Indeed, the sample 112 of the substrate of the semiconductor device 130 that is illuminated may be several mm² or larger. In these and other embodiments, the images 124 and 126 may be formed based on the light beams 119 a, 119 b reflected or scattered by the topographical features or thickness of the sample 112. Thus, the images 124 and 126 may be an image of a portion or may cover an entire area of the sample 112, as opposed to a limited portion of the semiconductor device 130.

In another embodiment, the digital imagers 114 and 116 may be line scan cameras scanning across a larger illuminated area on raised surface feature 113 a. By moving the semiconductor device 130 continuously across the field of view of the line scan camera, images 124 and 126 of an entire area of the surface may be gradually built and acquired. Alternatively still, the scan lines may be electrically steered to scan a surface or different adjacent scan lines may be activated in turn to scan the intended surface. Other embodiments may occur to those of skill in this art.

In these and other embodiments, particular pixels in the images 124 and 126 may correspond to particular locations in the area of the sample 112. The processor system 128 may be configured to determine a distance between the reference plate 110 and raised surface feature 113 a and a distance between the reference plate 110 and the surface 113 b of the sample 112 at multiple different locations within the area of the sample 112. In these and other embodiments, the processor system 128 may use illumination intensity information from particular pixels in the image 126 from the digital imager 116 on the interferometer channel 122 a to determine the distance between the raised surface feature 113 a and the floor surface 113 b at particular locations of the sample 112 that correspond to the particular pixels in the image 126.

For example, a first pixel or a first group of pixels in the image 126 may correspond to a portion of the reflected light beam 119 a that reflected from reference plate 110 and from a first location of the sample 112. A second pixel or a second group of pixels in the image 126 may correspond to a portion of the reflected light beam 119 a that reflected from reference plate 110 and from a second location of the sample 112. Thus, the first pixel in the image 126 may have a grayscale value that is based on the first pixel or on the first group of pixels in the image 126, based on the intensity of the reflected light beam 119 a that reflected from the first location of the sample 112. Furthermore, the second pixel in the image 126 may have a grayscale value that is based on the second pixel or on the second group of pixels in the image 126, based on the intensity of the reflected light beam 119 a that reflected from the second location of the sample 112.

In these and other embodiments, the processor system 128 may be configured to determine the distance between the reference plate 110, the raised surface feature 113 a and the floor surface 113 b at the first location of the sample 112 based on the grayscale value(s) of the first pixel or the first group of pixels. The processor system 128 may also be configured to determine the distance between the reference plate 110, the raised surface feature 113 a and the surface 113 b at the second location of the sample 112 based on the grayscale value(s) of the second pixel or the second group of pixels. In these and other embodiments, the distance between the reference plate 110 and the raised surface feature 113 a and the surface 113 b at the first location and the second location may be different. In these and other embodiments, based on the different distances between the raised surface feature 113 a and the floor surface 113 b from the reference plate 110 at different locations of the sample 112, the processor system 128 may generate a topology/topography representation or a data set that is representative of the topology/topography of the area of the sample 112 that reflects the different distances between the raised surface feature 113 a and the floor surface 113 b at different locations of the sample 112.

The different intensities of the reflected light beam 119 a received by different pixels of the digital imager 116 may result from different distances between the raised surface feature 113 a and the floor surface 113 b at different locations of the sample 112. The different distances between the raised surface feature 113 a and the floor surface 113 b at different locations of the sample 112 may result in different path length differences traversed by the first light beam reflection 121, the second light beam reflection 122 and the third light beam reflection 123 at different locations of the sample 112. The different path length differences may result in different phase differences between the reflections from at different locations. The different phase differences may result in a change in intensity when the second light beam reflection 122 and the third light beam reflection 123 add coherently with first beam reflection 121 from reference plate 110 that is coaxially-disposed with beam 121, to form the reflected light beam 119 a. These beams may add coherently and generate an intensity (grayscale) pattern that is dependent on the phase difference between the first light beam reflection 121 and second beam reflection 122 and the phase difference between the first light beam reflection 121 and the third light beam reflection 123. For example, when the reflected light beams are in-phase, they interfere constructively (strengthening in intensity). Conversely, when the reflected beams are out-of-phase, they interfere destructively (weakening in intensity). These changes in intensity differences may be represented by the different grayscale values of the pixels in the image 126.

An example of the operation of the system 100 a is now described. The tunable light source 102 may be configured to generate and to emit a number of different light beams 118 a, 118 b. Each of the multiple different light beams 118 a, 118 b may be generated at a different time and at a different wavelengths within predetermined wavelength ranges. In some embodiments, the different wavelengths of the different light beam 118 a, 118 b may result in different intensities of the reflected light beams 119 a, 119 b. The different intensities may be due to the different wavelengths of the different light beams 118 a, 118 b causing differences in the phase between the second light beam reflection 122 and the third light beam reflection 123 when added coherently to the first beam reflection 121 from the reference plate 110. For example, at a first wavelength of the light beam 118 a, the coherently added first light beam reflection 121 from reference plate 110 and the second light beam reflection 122 and the coherently added first light beam reflection 121 from reference plate 110 and the third light beam reflection 123 may have a first phase difference at a second wavelength of the light beam 118 a, the coherently added first light beam reflection 121 from reference plate 110 and the second light beam reflection 122 and the coherently added first light beam reflection 121 from reference plate 110 and the third light beam reflection 123 may have a second phase difference. The coherent addition with different phase differences may cause the reflected light beam 1198 a to have different intensities. According to one embodiment, the number of the light beams emitted by the tunable light source 102 may be selected based on the distance between the surface of the reference plate 110 and the raised surface features of the sample 112. The determined distances between the reference plate 110 that faces the sample 112 and the raised surface features of the sample 112 may be based on a plurality of images generated based on the plurality of light beams.

Each of the different reflected light beams 119 a may be used by the digital imager 116 to generate a different image 126. The processor system 128 may receive and store each of the different images generated by the digital imager 116. The processor system 128 may then use the different received images 126 to determine the distance between the raised surface feature 113 a and the floor surface 113 b.

In some embodiments, the processor system 128 may use the different intensities of the reflected beams 119 a of different images to determine the distance between the raised surface feature 113 a and the floor surface 113 b. For example, in some embodiments, the processor system 128 may extract the grayscale value, representing an intensity value, for a corresponding (e.g., same) pixel of each image 126. The corresponding pixel in each image 126 may correspond with a particular pixel element in the digital imager 116. Thus, a particular pixel in each image 126 may be generated from the same pixel element in the digital imager 116. The grayscale values for the particular pixel in each image 126 may be plotted to form a fringe pattern with a sinusoidal waveform, a modulated sinusoidal waveform, or a complex waveform. For example, the intensity values of a particular pixel from different images may be plotted along the y-axis and the wavelength of the light beam 118 a used to generate the different images may be plotted along the x-axis. In these and other embodiments, the distance between the reference plate 110 and the raised surface feature 113 a and the distance between the reference plate and floor surface 113 b at a particular point corresponding to the particular pixel may be determined based on the plotted fringe patterns.

For example, in some embodiments, the distance between the reference plate 110 and the raised surface feature 113 a and the distance between the reference plate 110 and floor surface 113 b at a particular point corresponding to a particular pixel may be determined based on a frequency domain transformation, such as a Fast Fourier Transform (FFT), a Discrete Fourier Transform (DFT), a Discrete Cosine Transform (DCT) or Hilbert Transform of the fringe patterns. Other transformations may be utilized. Alternately or additionally, in some embodiments, these distances at a particular point corresponding to a particular pixel may be determined based on a comparison between a model-based predicted fringe pattern and the determined pixel intensity fringe patterns from the images 126. Each of the model-based predicted fringe patterns may be constructed for a different distance, based on previous actual results or theoretical mathematical expressions. For example, a relationship between a phase difference and an intensity of reflected light beam 119 a may be determined or estimated using the following mathematical expression:

$I_{0} = {I_{1} + I_{2} + {2\sqrt{I_{1}I_{2}}{\cos\left( \frac{2\pi d}{\lambda} \right)}}}$

In the above expression, “I₁” refers to the intensity of the first light beam reflection 121 from reference plate 110, “I₂” may refer to the intensity of the second light beam reflection 122 from the raised surface feature 113 a, “d” may refer to the optical distance between the reference plate 110 and the raised surface feature 113 a, “A” refers to the wavelength of the light beam 118 a, and “I₀” may refer to the measured intensity of the reflected light beam 119 a. In the above expression, “I₂” may also refer to the intensity of the third light beam reflection 123 from the floor surface 113 b, “d” may refer to the optical distance between the reference plate 110 and the floor surface 113 b, “X” may refer to the wavelength of the light beam 118 a, and “I₀” may refer to the intensity of the reflected light beam 119 a by coherently adding the first light beam reflection 121 and the third light beam reflection 123. Based on the above expression, model based predicted fringe patterns may be created for determining the optical height of the feature on sample 130.

In these and other embodiments, the fringe pattern determined from processor system 128 may be compared to each or some of the model-based predicted fringe patterns. The model-based predicted fringe patterns closest to the determined fringe pattern may be selected and the distance for which the selected model based predicted fringe was constructed may be the determined distance between the raised surface feature 113 a and the floor surface 113 b.

In some embodiments, the processor system 128 may perform an analogous analysis for each pixel of the different images 126. Using the distance information from each pixel, the processor system 128 may determine the topology of the area of the sample 112 illuminated by the light beam 118 a.

In some embodiments, a number of different light beams 118 a with different wavelengths used by the system 100 a and thus a number of different images generated by the digital imager 116 may be selected based on an estimated distance between the raised surface feature 113 a and the floor surface 113 b. When the optical distance between the raised surface feature 113 a and the floor surface 113 b is small, such as below 1 micrometer (μm), the number of different light beams 118 a may be increased as compared to when the optical distance between the raised surface feature 113 a and the floor surface 113 b is larger, such as above 1 μm. In these and other embodiments, an inverse relationship between the distance to be determined between the two surfaces and the number of different light beams 118 a may exist. As such, a bandwidth of the wavelengths of the different light beams 118 a may have an inverse relationship with the distance to be determined between the raised surface feature 113 a and the floor or reference surface 113 b, as shorter distances may call for an increased number of different wavelengths to correctly characterize the small height of the raised surface features of the sample 112, and vice-versa.

The relationship between the distance to be determined between the raised surface feature 113 a and the floor surface 113 b and the difference in the wavelength between respective different light beams 118 a (the wavelength step size) may also have an inverse relationship. Thus, for a small size distance between the raised surface feature 113 a and the floor surface 113 b, the wavelength step-size may be a first wavelength step-size. For a medium size distance between the raised surface feature 113 a and the floor surface 113 b, the wavelength step-size may be a second wavelength step-size and for a large size distance between the raised surface feature 113 a and the floor surface 113 b, the wavelength step-size may be a third wavelength step-size. In these and other embodiments, the third wavelength step-size may be smaller than the first and second wavelength step-size and the second wavelength step-size may be smaller than the first wavelength step-size. Additionally, the bandwidth of each light beam 118 a corresponding to each wavelength step may get smaller as the distance between the raised surface feature 113 a and the floor surface 113 b increases.

In some embodiments, the semiconductor device 130 may be repositioned with respect to the system 100 a. For example, the semiconductor device 130 may be moved or the system 100 a may be moved. Both may be moved relative to one another. In these and other embodiments, the system 100 a may be configured to determine a distance between the raised surface feature 113 a and the floor surface 113 b from a second sample of the semiconductor device 130. The second sample of the semiconductor device 130 may have been a portion of the semiconductor device 130 that was previously unilluminated by the light beam(s) 118 a, 118 b or for which reflections from second sample did not reach the digital imagers 114 and 116. In these and other embodiments, the semiconductor device 130 may be repositioned such that entire surface of the semiconductor device 130 may be a sample for which the distance between the raised surface feature 113 a and the floor surface 113 b is determined or image of the sample surface is captured. In these and other embodiments, the system 100 a may be repositioned such that entire surface of the semiconductor device 130 may be a sample for which the distance between the raised surface feature 113 a and the floor surface 113 b is determined or image of the sample surface is captured.

Modifications, additions, or omissions may be made to the system 100 a. For example, in some embodiments, the system 100 a may include additional optical components between the first and second beam splitters 104 a, 104 b and the digital imagers 114 and 116, as illustrated in FIG. 2.

The system 100 a as shown and described herein differs from previous distance measurement concepts. Advantageously, according to embodiments, because the reference plate 110 and both the raised surface feature 113 a and the surface 113 b are illuminated by the same light beam 118 a, any vibrations of the semiconductor device 130 affects all beam reflected from sample 130 substantially equally and at substantially the same time such that the system 100 a may compensate for the vibrations.

In some embodiments, an interferometer system may include multiple tunable light sources, multiple beam splitters, and multiple digital imagers. In some embodiments, an interferometer system may include single tunable light sources, multiple beam splitters, and digital imagers. In these and other embodiments, a tunable light source, a beam splitter, and a digital imager may be referred to in this disclosure as interferometer sub-systems.

FIG. 1A illustrates an embodiment of an inspection system 100 a (the “system 100 a”). In general, the system 100 a may be configured to image a raised surface feature 113 a of a sample 112 that is part of a semiconductor device 130 using light beams 118 a and 118 b. To image the surface, the system 100 a may include a tunable light source 102, beam splitters 104 a and 104 b, lens system 106, a digital imager 114, and a processor system 128.

The system 100 a may be implemented to generate an image of the raised surface features 113 a and floor surface 113 b by illuminating the surfaces 113 a, 113 b with light beams 118 a and 118 b. The two beams may be generated simultaneously or may be generated one after another.

In some embodiments, the imaging system 100 a may include light beam 118 b, an off-axis ring illuminator 108, a second beam splitter 104 b and a digital imager 114. This embodiment enables generation of dark field images of the sample surface 113 a. The off-axis ring illuminator 108, as shown in FIG. 1, transmits the light beam 118 b toward the sample 112 at an angle (other than normal) relative to the reference plate 110 and the sample 112, as opposed to the normal incidence light beam 118 a, which is directed toward the reference plate 110 and the sample 112 perpendicularly, or substantially perpendicularly, as shown in FIGS. 1A, 2 and 3.

In some embodiments, the imaging system 100 a may include light beam 118 a, first and second beam splitters 104 a and 104 b, imaging lens system 106 and digital imager 114. This embodiment enables generation of brightfield field images of the surface of the sample surface.

In other embodiments, the imaging system 100 a may include both light beams 118 a and 118 b, both first and second beam splitters 104 a and 104 b, and a digital imager 114. This embodiment enables generation of brightfield field images of the surface of the sample.

In still further embodiments, the imaging system 100 a may include both light beams 118 a and 118 b, both first and second beam splitters 104 a and 104 b, and both digital imagers 114 and 116. This embodiment enables generation of brightfield field images of the surface of the sample.

The system 100 a may be implemented to generate a fluorescence image of the surface 113 a. In some embodiment, a fluorescence image of the surface 113 a is generated by illuminating the surface 113 a with light beams 118 a and 118 b composed of one or more shorter wavelengths from the tunable source 102. This shorter wavelength may excite materials of surface 113 a to fluoresce at correspondingly longer wavelengths. The fluorescence emission from surface 113 a may be imaged by both digital imagers 114 and 116.

In some embodiment, a fluorescence image and bright field/dark field image of the surface 113 a may be generated by illuminating the surface of the sample with light beams 118 a and 118 b composed of shorter wavelengths from the tunable source 102. This shorter wavelength may excite materials of surface to fluoresce at correspondingly longer wavelengths. The fluorescence emission along with reflected and scattered light from the surface of the sample may be imaged by both digital imagers 114 and 116, thereby enabling darkfield inspection, brightfield inspection and fluorescence inspection to occur simultaneously in a single device. For example, the tunable light source 102 may be configured to generate a first light beam, 118 a having a wavelength of, in this example, 520 nm. Interaction of this wavelength of light with the sample 112 and the raised features thereof causes emission of fluorescence light at a comparatively longer wavelength, such as 620 nm in this example. The 620 nm light, as well as that portion of the 520 nm light that did not cause fluorescence, are directed back through the lens system 106, through the first beam splitter 104 a toward the second beam splitter 104 b. In this embodiment, the second beam splitter 104 b may be a dichroic mirror configured to primarily direct the fluorescence signal at 520 nm to the imaging channel 122 b to imager 114 for fluorescence imaging and inspection and to primarily direct the reflected 520 nm (with phase change) light beam to digital imager 116 for brightfield inspection, or vice versa. A third digital imager and a third dichroic mirror may be added to primarily direct reflections from the second light beam 118 b to third digital imager for darkfield inspection of the surface features of the sample 112, to thereby enable brightfield, darkfield and fluorescence inspection simultaneously, in one device.

The system 100 a may be implemented with respect to any suitable application where a surface inspection may be required. For example, in some embodiments, the raised surface feature 113 a may be or comprise a bump feature of a semiconductor advanced packaging device 130. In these and other embodiments, the semiconductor device 130 may be any circuit, chip, device, or 3D feature that is fabricated on a silicon wafer. The semiconductor device 130 may include multiple layers of the same or different materials between the raised surface feature 113 a and the surface 113 b. Alternately or additionally, the raised surface feature 113 a may be a MEMS structure and the surface 113 b may be a surface on which the MEMS structure is built.

Alternately or additionally, the raised surface feature 113 a may be any type of interconnect feature used in 3D packaging and the surface 113 b may be the corresponding surface from which the interconnect features protrude. An example of a protruding feature and a reference surface is described with respect to FIG. 4. Alternately or additionally, the raised surface feature 113 a may be an embedded surface within a semiconductor device or some other device and the reference surface may be a top surface as shown in FIG. 5. Although FIGS. 1, 2, 4 and 5 illustrate certain surface feature configurations, the principles and operation of the systems described in FIGS. 1, 2, 4 and 5 may be applied to any surface feature configuration.

The digital imagers 114 and 116 may be configured to receive the reflected light beam 119 a, 119 b and to correspondingly generate respective images 124, 126 based on an intensity of the respective reflected light beams 119 a, 119 b. In some embodiments, the digital imagers 114, 116 may be CMOS or CCD type imager or other types of 2D- and 1D-array detectors. In these and other embodiments, these digital imagers may include multiple pixel elements. The pixel elements may be configured such that, when illuminated, each pixel element provides information about the intensity of the illumination that is striking the pixel element. The digital imagers 114, 116 may compile the information from the pixel elements to form the images 124, 126, respectively. These images may thus include the intensity information for each of the pixel elements. The images 124, 126, when including the intensity information for each pixel element, may be referred to as a grayscale digital images. The digital imager 114 may provide the image 124 to the processor system 128 and digital imager 116 may provide the image 126 to the processor system 128.

FIG. 1B illustrates multiple interferometer sub-systems 160 a and 160 b in an exemplary interferometer system 100 b, arranged according to at least some embodiments described in this disclosure. Each of the sub-systems 160 a and 160 b may include a tunable light source, one or more beam splitters, lens system and digital imagers analogous to the tunable light source 102, the first and second beam splitters 104, lens system 106 and the digital imagers 114, 116 of FIG. 1A. Each of the sub-systems 160 a and 160 b may be configured to illuminate a different portion of a semiconductor device or other sample 180. Images generated by each of the sub-systems 160 a and 160 b may be provided to a processor system 190 that may be analogous to the processor system 128 of FIG. 1A. The processor system 190 may be configured to determine height of the surface features of the semiconductor device 180 based on the images from the sub-systems 160 a and 160 b. The processor system 190 may be configured to generate an image of the surface of sample 180. Thus, in these and other embodiments, multiple samples of the semiconductor device 130 may be processed at the same time, in parallel. By processing multiple samples at the same time, a height of the raised surface features across the semiconductor device 180 may be determined in less time than when portions of the semiconductor device 180 are processed sequentially or one at a time. Similarly, by processing multiple samples simultaneously, images across the semiconductor device 160 may be generated in less time than when successive portions of the semiconductor device 180 are processed sequentially, one at a time.

Modifications, additions, or omissions may be made to the system 100 b without departing from the scope of the present disclosure. For example, each of the sub-systems 160 a and 160 b may include a processor system. In these and other embodiments, one of the processor systems may compile information for the entire semiconductor device 180 from other of the processor systems.

FIG. 2 illustrates another example interferometer system 200 a (the “system 200 a”), according to at least some embodiments described in this disclosure. The system 200 a may be configured, using light beams 218 a, 218 b, to determine a height of a raised surface feature 213 a from floor surface 213 b on a sample 212 that is part of a semiconductor device 230. To determine the distance or capture an image, the system 200 a may include a tunable light source 202, first and second beam splitters 204 a, 204 b, a first lens system 206, digital imagers 224 and 226, and a processor system 228.

The system 200 a may be implemented with respect to any suitable application where a distance may be measured. For example, in some embodiments, the raised surface feature may be a top surface of a semiconductor device 230 and the floor surface 213 b may be a top surface of a silicon substrate wafer that forms a substrate of the semiconductor device 230 from which the raised surface features rise.

The tunable light source 202 may be configured to generate and to emit a light beam 218 a and light beam 218 b. The tunable light source 202 may be analogous to the tunable light source 102 of FIG. 1A and may be configured to provide light beams 218 a, 218 b at a particular wavelength or at several different wavelengths within a range of wavelengths over a period of time. As illustrated in FIG. 2, in some embodiments, the tunable light source 202 may include a broadband light source 222 and a tunable filter 210 that are co-axially disposed and optically coupled. The broadband light source 222 may be configured to emit a broadband light beam 216 that includes wavelengths of light that may be used by the system 200 a. In some embodiments, the broadband light source 222 may be a light source such as a white light or a super luminescent diode (SLED). In some embodiments, the broadband light source 222 may be configured to provide the broadband light beam 216 with a Gaussian power spectrum.

The tunable filter 210 may be configured to filter the broadband light beam 216 to generate the light beam 218 a at a particular wavelength. In some embodiments, the tunable filter 210 may be tuned, such that the tunable filter 210 may filter different wavelengths of light to generate the light beam 218 a at multiple different wavelengths of light.

An off-axis ring illuminator 208 may be provided and receive light beam 218 b also generated by broadband light source 222 to illuminate the sample 212 with off-axis (e.g., not normal incidence) light beam 218 b. In some embodiments, the first beam splitter 204 a may be configured to receive the light beam 218 a and to direct the light beam 218 a, towards the sample 212 via lens system 206. The first beam splitter 204 a may be further configured to reflect and transmit a portion of the light beam 218 a, 218 b. For example, the first and second beam splitters 204 a, 204 b may reflect 50 percent and transmit 50 percent of the light beams 218 a, 218 b. Alternately or additionally, the beam splitters 204 may reflect a different percent of the light beams 218 a, 218 b (such as to reflect all or substantially all of the first light beam 218 a). In these and other embodiments, the reflected portion of the light beam 218 a reflected by first beam splitter 204 a may be directed to the sample 212.

The sample 212 may be analogous to the sample 112 in FIG. 1A. In these and other embodiments, the light beams 218 a, 218 b may be reflected and/or scattered by the raised surface feature 213 a and the floor surface 213 b of the sample 212 to form the reflected light beams 218 a, 218 b. The reflected light beams 218 a, 218 b may be received by the first and second beam splitters 204 a, 204 b via first lens system 206. The first and second beam splitters 204 a, 204 b may reflect a portion and transmit a portion of the reflected light beams 218 a, 218 b.

The first lens system 206 may be configured to receive the reflected light beams 218 a and 218 b from sample 212 and to focus images of the sample on the digital imagers 224, 226. A second lens system 207 may be provided between the first and second beam splitters 204 a, 204 b. The first lens system 206 and the second lens system 207, if present, may pass and focus the reflected light beam 218 a, 218 b toward the digital imagers 224 and 226. The digital imagers 224, 226 may include an image sensor. The image sensor may include a CMOS image sensor, a CCD image sensor, or other types of 1D- and 2D-array detectors. The digital imagers 224 and 226 may generate images 234 and 236 based on the reflected and scattered light beams 218 a, 218 b and pass the images to the processor system 228.

The processor system 228 may be analogous to and configured to operate in a similar manner as does the processor system 128 of FIG. 1A. The processor system 228 may be implemented by any suitable mechanism, such as a program, software, function, library, software as a service, analog, or digital circuitry, or any combination thereof. In some embodiments, such as illustrated in FIG. 2, the processor system 228 may include a data acquisition module 250 and a processor and storage module 252. The processor and storage module 252 may include, for example, a microprocessor, microcontroller, digital signal processor (DSP), application-specific integrated circuit (ASIC), a Field-Programmable Gate Array (FPGA), or any other digital or analog circuitry configured to interpret and/or to execute program instructions and/or to process data. In some embodiments, the processor of the processor and storage module 252 may interpret and/or execute program instructions and/or process data stored in the memory of the processor and storage module 252. For example, the images 234 and 236 generated by the digital imagers 224 and 226 may be stored in the memory of the processor and storage module 252. The processor of the processor and storage module 252 may execute instructions to perform the operations with respect to the image 234 and 236 to generate image of sample 212 and to determine the distance between the raised surface feature 213 a and the floor surface 213 b and/or thickness of the sample 212

The memory in the processor and storage module 252 may include any suitable computer-readable media configured to retain program instructions and/or data, such as the images 234 and 236, for a period of time. By way of example, and not limitation, such computer-readable media may include tangible and/or non-transitory computer-readable storage media including Random Access Memory (RAM), Read-Only Memory (ROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), Compact Disc Read-Only Memory (CD-ROM) or other optical disk storage, magnetic disk storage or other magnetic storage devices, flash memory devices (e.g., solid state memory devices), or any other storage medium which may be used to carry or store desired program code in the form of computer-executable instructions or data structures and which may be accessed by a general-purpose or special-purpose computer. Combinations of the above may also be included within the scope of computer-readable media. Computer-executable instructions may include, for example, instructions and data that cause a general-purpose computer, special-purpose computer, or special-purpose processing device to perform a certain function or group of functions. Modifications, additions, or omissions may be made to the system 200A without departing from the scope of the present disclosure.

The embodiment of the interferometer system 200 a of FIG. 2 may include an adjustable aperture device 232, which may be adjusted, according to one embodiment, based upon the lateral resolution of the interferometer system 200 a.

The adjustable aperture device 232 may be configured to adjust a size of an aperture 242 through which the reflected light beams 219 a, 219 b may travel. In some embodiments, the adjustable aperture device 232 may be positioned between the first and second beam splitters 204 a and 204 b. The adjustable aperture device 232 may also be positioned between the first lens system 206 and the first beam splitter 204 a. Alternately or additionally, as shown in FIG. 2, the adjustable aperture device 232 may be positioned between the first lens system 206 and tube lenses of the digital imagers 224 and 226. In some embodiments, the aperture 242 of the adjustable aperture device 232 may result in an adjustable system pupil plane. A position of the adjustable system pupil plane may be based on a position of the adjustable aperture device 232 (and therefore the size of aperture 242) in the imaging path made up of lens system 206, aperture 242, and the digital imagers 224, 226. In some embodiments, a position of the adjustable system pupil plane, and thus the position of the adjustable aperture device 232, may be determined based on whether the adjustable system pupil plane is configured to control spatial resolution or field of view of the digital imagers 224, 226.

In some embodiments, the size of the aperture 242 may be adjusted based on a feature size in an area of the sample 212. In some embodiments, the size of the aperture 242 may be adjusted based on a required spatial resolution of the area of the sample 212 that is being imaged by the digital imagers 224, 226. In these and other embodiments, adjusting the size of the aperture 242 may affect one or more of: a light beam angle or a numerical aperture (NA) of the reflected light beam 219 a or 219 b on the first lens system 206; relay of the reflected light beam 219 a, 219 b; sharpness of the images 234, 236 generated by the digital imagers 224, 226; depth of focus, the field of view and spatial resolution on the digital images, among others.

In some embodiments, the system 200 a may be configured before determining a distance between the raised surface feature 213 a and the surface 213 b. In these and other embodiments, the size of the aperture 242 may be selected. The size of the aperture 242 may be selected based on an area of the sample 212. The area of the sample 212 may be an area in a plane that includes at least a portion of a raised surface feature 213 a. In these and other embodiments, the size of the aperture 242 may be based on required lateral resolution, i.e., the smallest size of a feature to be measured within or on the sample 212 of the semiconductor device 230. In these and other embodiments, when the lateral size of the feature is small, the size of the aperture 242 is correspondingly larger. When the lateral size of the feature is larger, the size of the aperture 242 is correspondingly smaller.

In some embodiments, configuring the system 200 a may include setting an exposure time and gain of the digital imagers 224 and 226. In these other embodiments, an initial exposure time and gain may be selected for the digital images 234, 236. The initial exposure time and gain may be selected based on the area and the reflectivity of the sample 212. In one embodiment, the exposure time, and the signal to noise of the digital imagers 224, 226 may be dependent on digital imagers' read-out time, the number of wavelengths in the plurality of light beams and the reflectivity of the sample 212.

After selecting the initial exposure time and gain, the light beams 218 a, 218 b may illuminate the sample 212 and images 234, 236 may be captured by the digital imagers 224 and 226 from the reflected light beams 219 a, 291 b. The images 234, 236 may be processed to determine if any pixels thereof saturated after having been exposed to the reflected light beam 219 a, 219 b. Saturation may be determined when there is flat line of a grayscale value across multiple adjacent pixels in the images 234, 236. When it is determined that some of the pixels of the images 234, 236 saturated, the gain and/or the exposure time may be reduced. For example, the gain may be reduced by ten percent. The process of checking for saturation by the processor system 228 may be iteratively carried out and the gain and the exposure time successively reduced until the little or no saturation of pixels occurs at a particular wavelength of the light beams 218 a, 218 b. In these and other embodiments, the particular wavelength selected may be the wavelength with the highest power. Using the wavelength with the highest power during configuration may reduce the likelihood of saturation of pixels with wavelengths of lower power during operation of the system 200 a.

In some embodiments, configuring the system 200 a may include selecting a range of wavelengths for the light beams 218 a, 218 b and the wavelength step size between light beams 218 a, 218 b. In some embodiments, the range of wavelengths for the light beams 218 a, 218 b and the wavelength step size may be selected based on a shortest distance between the raised surface feature 213 a and the floor surface 213 b over the area of the sample 212. In these and other embodiments, an approximate or estimated shortest distance may be selected based on the known design and construction of the semiconductor device 230. Indeed, the intended, designed-for smallest and largest features of the semiconductor device 230 may be known a priori. In these and other embodiments, the range of wavelengths for the light beams 218 a, 218 b and the wavelength step size may then be selected based on the shortest anticipated feature size. As discussed previously, the range of wavelengths for the light beams 218 a, 218 b and the wavelength step size may have an inverse relationship with respect to distance between the raised surface feature 213 a and the floor surface 213 b.

Modifications, additions, or omissions may be made to the system 200 a without departing from the scope of the present disclosure. For example, in some embodiments, the adjustable aperture device 232 may be located between the first lens system 206 and the first beam splitter 204 a or between the first and second beam splitters 204 a, 204 b, as shown in FIG. 2.

FIG. 4 illustrates an example of beam reflection off another exemplary semiconductor device 400, arranged in accordance with at least some embodiments described in this disclosure. As shown therein, reference plate 410 is positioned facing a distance away from and above the semiconductor device 400. According to embodiments, the reference plate 410 may be coated for partial reflectance and partial transmissivity. The semiconductor device 400 may include a first raised portion 406 a and a second raised portion 406 b, both of which extend above a sample floor surface 404 of the semiconductor device 400. A top surface of the first raised portion 406 a may be a first raised surface feature 402 a of the semiconductor device 400. A top surface of the second raised portion 406 b may be a second raised surface feature 402 b of the semiconductor device 400. Using light beams and an interferometer system described in some embodiments in the present disclosure, a distance D1 between the reference plate 410 and the first feature surface 402 a may be determined. Alternately or additionally, a distance D2 between the reference plate 410 and the surface 404 may be determined. From these two distance measurements, the height of the raised surface feature 406 a away from surface 404 may be determined. In some embodiments, the floor surface 404 may be at varying heights with respect to the first and second raised portions 406 a and 406 b. For example, the floor surface 404 to the left of the first raised portion 406 a may be higher than the floor surface 404 to the right of the second raised portion 406 b. Therefore, the height of any raised surface feature of the semiconductor device 400 may be characterized by its height above a stated reference surface. Indeed, that reference surface may be the underside of the reference plate that faces the semiconductor device 400 as shown with reference to D1. Alternatively, that reference surface may be a predetermined portion of the sample floor surface 404, as shown with reference to D2.

FIG. 4 illustrates a light beam from a light source TS1. The light beam from TS1 includes a first light beam portion 414 a that is incident upon the reference plate surface 410, thereby generating first reflected beam 415 a and a second light beam portion 414 a′ that strikes the surface 404. A part of the second light beam portion 414 a′ may be reflected off of the surface 404 and generate a second reflected beam 416 a. The rest of the first light beam portion 414 a′ may pass through the semiconductor device 400 and/or incur additional reflections, scattering or refractions.

A part of the second light beam portion 414 b from a light source TS2 may be reflected off of the reference plate to generate reflected beam 415 b and a part of the second light beam portion 414 b may strike the raised surface feature 402 a and generate a second reflected beam 416 b. A remaining portion of the second light beam portion 414 b′ may pass through the semiconductor device 400 and/or incur additional reflections, scattering or refractions.

The first and second reflected beams 415 a, 416 a and 415 b, 416 b may coherently add to form a combined reflected beam 420. In some embodiments, the reflected beam 420 may pass through the first lens system 206, the aperture 242 as illustrated and described with respect to FIG. 2 and be provided to the digital imagers 224, 226. Images (such as shown at 234, 236 in FIG. 2, for example) may be formed using at least the intensity of the reflected beam 420. The images thus generated may be part of a collection of images that may be stored and used to determine the distances D1 and D2.

In some embodiments, the light source TS1 may also illuminate the second raised portion 406 b. In a similar manner as described with respect to the first raised portion 406 a, a reflected beam may be formed and captured to form an image or images. The image may be part of a collection of images that may be used to determine the distances D1 and D2.

FIG. 5 illustrates an illustrative example of beam reflection off another exemplary semiconductor device 500, arranged in accordance with at least some embodiments described in this disclosure. The semiconductor device 500 may include a first raised portion 506 a and a second raised portion 506 b that extend away from a first surface 508 of the semiconductor device 500 toward a reference plate 504. A top surface of the first raised portion 506 a may be a first raised surface feature 502 a of the semiconductor device 500. A top surface of the second raised portion 506 b may be a second raised surface feature 502 b of the semiconductor device 500. Using light beams and an interferometer system described in some embodiments in the present disclosure, a distance D1 between the reference plate 504 and the first feature surface 502 a may be determined. The distance D1 may represent a distance that the first feature surface 502 a is below (at least in the orientation shown in FIG. 5) the reference plate 504. Alternately or additionally, a distance D2 between the reference plate 504 and the second raised surface feature 502 b may be determined. In some embodiments, the reference plate 504 may be at varying heights with respect to the first and second raised portions 506 a and 506 b or with respect to the reference plate 504. In some embodiments the reference plate may not be part of the semiconductor device 500 or part of substrate 508.

FIG. 5 illustrates a light beam 514 a generated from a light source TS1. The light beam 514 a may strike the reference plate 504. A part of the light beam 514 a may be reflected off the reference plate 504 and generate a first reflected beam 516 a. The rest (or some remaining portion) of the light beam 514 a may pass through the reference plate 504 and generate a refracted beam 514 b. The refracted beam 514 b may hit the first raised surface feature 502 a of the first raised portion 506 a and part of the refracted beam 514 b may be reflected off the first features surface 502 a to generate a second reflected beam 516 b. The second reflected beam 516 b may pass through the reference plate 504, although some additional reflections, refractions and scattering may also occur.

The first and second reflected beams 516 a and 516 b may coherently add to form a reflected beam 520. In some embodiments, the reflected beam 520 may pass through the first lens system 206, and the aperture 242, as illustrated and described with respect to FIG. 2 and be provided to the digital imagers 224, 226. Images (such as shown at 234, 236, for example) may be formed using at least the intensity of the reflected beam 520. The thus formed images may be part of a collection of images that may be used to determine the distance D1.

In some embodiments, the light source TS1 may also illuminate the second raised portion 506 b. In a similar manner as described with respect to the first raised portion 506 a, a reflected beam may be formed and captured to form an image or images. These images may be part of a collection of images that may be used to determine the distance D2.

One skilled in the art will appreciate that, for this and other processes and methods disclosed in this disclosure, the functions performed in the processes and methods may be implemented in differing order. Furthermore, the outlined steps and operations are only provided as examples, and some of the steps and operations may be optional, combined into fewer steps and operations, or expanded into additional steps and operations within the scope of the disclosed embodiments.

For example, in some embodiments, the implementation method may include adjusting a size of an aperture (see reference 242 in FIG. 2, for example), through which the reflected light beam passes, based on an area of a feature along the feature surface for which the distance between the reference plate and the feature surface is determined.

In these and other embodiments, a wavelength difference between the first wavelength and the second wavelength emitted simultaneously or over time may be selected based on the height or depth of the raised surface feature on the sample surface relative to the reference plate 110 or the floor of the sample 112. The pathlength inside the reference plate 110 is not significant and both facing surfaces of the reference plate contribute to that pathlength. In these and other embodiments, determining the height or depth of raised surface features on sample surface based on wavelength dependent images may include constructing a waveform or fringe pattern based on wavelength dependent intensity values and performing a frequency domain transformation such as a Fast Fourier Transform or Hilbert Transform on the waveform or fringe pattern. In these and other embodiments, the distance between the different surfaces at a first location on the sample may be determined based on a first intensity value at a first pixel location in the first image and a second intensity value at the first pixel location in the second image. In these and other embodiments, the distance may be a first distance and the implementation method may further include determining a second distance between the reference plate and the raised surface feature based on the first image and the second image at a second location on the sample. The second distance may be determined based on a first intensity value at a second pixel location in the first image and a second intensity value at the second pixel location in the second image.

In these and other embodiments, the imaging lens system 106 shown in FIG. 1A or the imaging lens system 206 in FIG. 2 is common for both the imaging channel 122 b and the interferometer channel 122 a. Alternately or additionally each channel 122 a, 122 b or 222 a, 222 b may have its own, independent imaging lens system.

FIG. 3 illustrates another embodiment of an interferometer system 300 a (the “system 300 a”). The system 300 a may be configured to determine, using light beams, a height of a raised surface feature 313 a away from a floor or reference surface 313 b of a sample 312 that is part of a semiconductor device 330. To determine the distance or to capture an image, the system 300 a may include a tunable light source 302, first and second beam splitters 304 a, 304 b, a first lens system 306, an auto-correlation interferometer 340, digital imagers 334 and 336, and a processor system 338.

The system 300 a may be implemented with respect to any suitable application where a distance may be measured. For example, in some embodiments, the raised surface feature 313 a may be a top surface of a semiconductor device 330 and the floor surface 313 b may be a top surface of a silicon substrate wafer that forms a substrate of the semiconductor device 330.

The tunable light source 302 may be configured to generate and to emit light beams 318 a, 318 b. The tunable light source 302 may be analogous to the tunable light source 102 of FIG. 1A and may be configured to provide a light beam 318 a at a particular wavelength. An off-axis ring illuminator 308 may be provided and receive light beam 318 b to illuminate the sample 312 with off-axis light beam 318 b. As illustrated in FIG. 2, in system 300 a and in some embodiments, the tunable light source 302 may include a broadband light source and a tunable filter that are co-axially-disposed and optically coupled. The broadband light source may be configured to emit a broadband light beam that includes wavelengths of light that may be used by the system 300 a. In some embodiments, the broadband light source may be a light source such as a white light or a super luminescent diode (SLED). In some embodiments, the broadband light source may be configured to provide the broadband light beam with a Gaussian power spectrum.

The tunable light source 302 may be configured to generate the light beam 318 a at a particular wavelength. In some embodiments, the tunable light source 302 may be tuned, to generate different wavelengths of light to generate the light beam 318 a at multiple different wavelengths of light within a predetermined range of wavelengths. The tunable light source 302 also may be configured to generate the light beam 318 b at a particular wavelength. In some embodiments, the tunable light source 302 may be tuned to generate different wavelengths of light to generate the light beam 318 b at multiple different wavelengths of light within a predetermined range of wavelengths. These different wavelengths may be provided to the off-axis ring illuminator 308 over a period of time, to enable, among other applications, dark-field images of the sample 312.

In some embodiments, at least the first and beam splitters 304 a may be configured to receive the light beam 318 a and to direct the light beam 318 a towards the sample 312 via first lens system 306. The first and second beam splitters 304 a, 304 b may be configured to reflect and transmit a portion of the light beam 318 a, 318 b. For example, the first and second beam splitters 304 a, 304 b may reflect 50 percent and transmit 50 percent of the light beam 318 a. Alternately or additionally, the first and second beam splitters 304 a, 304 b may reflect a different percentage of the light beam 318 a, 318 b incident thereon. In these and other embodiments, the portion of the light beam 318 a reflected by the first beam splitter 304 a may be directed to the sample 312.

The sample 312 may be analogous to the sample 112 in FIG. 1A. In these and other embodiments, the light beams 318 a, 318 b may be reflected by the reference plate 310, the raised surface feature 313 a and the floor surface 313 b of the sample 312 to form the reflected light beams 319 a, 319 b. According to embodiments, the reference plate 310 may be coated for partial reflectance and partial transmittance. The reflected light beams 319 a 319 b, reflected and/or scattered by sample 330, may be received by the first and second beam splitters 304 a and 304 b via first lens system 306. A second lens may be provided between the beam splitters 304 a, 304 b. The first beam splitter 304 a may be configured to transmit at least a portion of the reflected light beams 319 a, 319 b towards the second beam splitter 304 b. The second beam splitter 304 b, in turn, may reflect a portion and transmit a portion of the reflected light beam 319 a. The reflected beam 319 a reflected by second beam splitter 304 b, in this embodiment, is directed toward an auto-correlation interferometer 340 over an auto correlation channel 323 to implement auto-correlation of reflections from the reference plate 310, and the surfaces 313 a and 313 b. The digital imager 334 is also coaxially disposed along the auto correlation channel 323 and is configured to receive light transmitted by the auto correlation interferometer 340 through the second beam splitter 304 b. In the system 300 a, the auto-correlation interferometer 340 may include a cavity having movable mirrors 341, 342. The mirrors 341 or 342 of auto-correlation interferometer 340 are movable along the optical axis such that the path differences between reflected light signal from reference plate 310 and those light reflections from surfaces 313 a and 313 b satisfy temporal coherence condition. By translating the mirror 341 and/or mirror 342 disposed in the auto-correlation interferometer 340, the reference plate reflection from moving the mirrors 341, 342 can be correlated to the fixed mirror reflections from various surfaces in the sample 330.

The first lens system 306 may be configured to receive the reflected light beam 319 a and the light beam from the reflected and/or scattered off-axis light beam 319 b off of the sample 312 through the first beam splitter 304 a. The reflected light beams 319 a, 319 b are then transmitted towards the second beam splitter 304 b, which reflects a portion thereof as an auto-correlator beam 319 a on the auto correlation channel onto the digital imagers 334 and a remaining portion thereof through the imaging channel 322 onto the digital imager 336. The digital imagers 334, 336 may include image sensors. The image sensor may be a CMOS image sensor, a CCD image sensor, or other types of 1D- and 2D-array detectors. The digital imagers 334 and 336 may generate images 324 and 326 based on the reflected and scattered light beams 319 a, 319 b and pass the images to the processor system 338.

The processor system 338 may be analogous to and configured to operate in a similar manner as the processor system 128 of FIG. 1A. The processor system 338 may be implemented by any suitable mechanism, such as a program, software, function, library, software as a service, analog, or digital circuitry, or any combination thereof. In some embodiments, such as illustrated in FIG. 2, the processor system 228 may include a data acquisition module 250 and a processor and storage module 252. The processor and storage module 252 may include, for example, a microprocessor, microcontroller, digital signal processor (DSP), application-specific integrated circuit (ASIC), a Field-Programmable Gate Array (FPGA), or any other digital or analog circuitry configured to interpret and/or to execute program instructions and/or to process data.

Terms used in this disclosure and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including, but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes, but is not limited to,” etc.).

Additionally, if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations.

In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” or “one or more of A, B, and C, etc.” is used, in general such a construction is intended to include A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B, and C together, etc. For example, the use of the term “and/or” is intended to be construed in this manner.

Further, any disjunctive word or phrase presenting two or more alternative terms, whether in the description of embodiments, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” should be understood to include the possibilities of “A” or “B” or “A and B.”

All examples and conditional language recited in this disclosure are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. Although embodiments of the present disclosure have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the present disclosure. 

1. A method, comprising: emitting a first light beam of a first wavelength at a first time; directing the first light beam toward a reference plate and a first sample facing the reference plate, the first sample defining a floor surface and a raised surface feature rising above the floor surface; generating a first image based on a first reflected light beam, the first reflected light beam being a coherent addition of a first reflection of the first light beam off of a surface of the reference plate, a second reflection of the first light beam off of the raised surface feature and a third reflection of the first light beam off of the floor surface; and based on the generated first image, determining a distance between the reference plate and the raised surface feature and determining a distance between the surface of the reference plate and the floor surface.
 2. The method of claim 1, further comprising: emitting a second light beam of a second wavelength at a second time different from the first time; directing the second light beam toward the first sample; and generating a second image based on a reflected second light beam, the reflected second light beam being a coherent addition of a first reflection of the second light beam off of the reference plate, a second reflection of the second light beam off of the raised surface feature and a third reflection of the second light beam off of the floor surface; based on the generated first and second images, determining the distance between the reference plate and the raised surface feature, and a distance between the reference plate and the floor surface.
 3. The method of claim 2, further comprising: selecting a wavelength difference between the first wavelength and the second wavelength based on a distance between the floor surface and the raised surface feature; and selecting a first bandwidth of the first light beam and a second bandwidth of the second light beam based on a distance between the reference plate and different raised surface features on the first sample.
 4. The method of claim 2, wherein determining the distance between the reference plate and the raised surface feature based on the first image and the second image includes: constructing a fringe pattern based on first intensity values of pixels from the first image of and second intensity values from pixels of the second image; and performing a frequency domain transform on the constructed fringe pattern.
 5. The method of claim 1, wherein determining the distance between the reference plate and the raised surface feature based on the first image comprises: comparing an intensity of a pixel of the image to a plurality of model-based pixel intensities that correspond with respective different distances; and selecting one of the plurality of model-based pixel intensities that is closest to the intensity of the pixel, wherein the determined distance is the distance corresponding to the selected one of the plurality of model-based pixel intensities.
 6. The method of claim 1, wherein the first sample is part of a semiconductor formed on a wafer, and wherein the method further comprises: emitting a second light beam; directing the second light beam toward a second sample of the semiconductor, the second sample being unilluminated by the first light beam and located on a different part of the semiconductor than the first sample; generating a second image based on a reflected second light beam, the reflected second light beam being a coherent addition of a first reflection of the second light beam off of the reference plate, of a second reflection of the second light beam off of a raised surface feature on the second sample and a third reflection of the second light beam off of the floor surface of the second sample; and based on the second image, determining second distances between the reference plate and the raised surface feature of the second sample, and between the reference plate and a floor surface of the second sample.
 7. The method of claim 1, wherein directing the first light beam comprises directing the first light beam toward the reference plate and the first sample at an angle normal to the reference plate.
 8. The method of claim 1, further comprising: emitting a second light beam; directing the second light beam toward the reference plate and the first sample at angles away from normal to the floor surface of the sample; generating a second image based on a second reflected light beam, the second reflected light beam being a coherent addition of a second reflection/scatter of the second light beam off of the reference plate, a second reflection/scatter of the second light beam off of the raised surface feature and a third reflection/scatter of the second light beam off of the floor surface; based on the generated second image, determining the distance between the reference plate and the raised surface feature and determining the distance between the reference plate and the floor surface.
 9. An optical system configured to measure raised surface features on a surface of a sample, comprising: a first digital imager and a second digital imager; a processor system coupled to the first and to the second digital imagers; a tunable light source configured to emit a first light beam and a second light beam; a first lens system configured to focus incident light onto the sample surface and to focus reflected light back on the first and second digital imagers; a reference plate disposed above and facing the sample surface; an off-axis ring illuminator and configured to receive and direct the second light beam toward the reference plate at angles other than normal to the sample surface; a first beam splitter configured to direct the first light beam through the first lens system toward the reference plate in a direction normal to the sample surface and to transmit a first reflected light beam of the first light beam reflected by the reference plate and a second reflected light beam of the second light beam reflected by a raised surface feature on the sample; a second beam splitter configured to transmit at least a portion of the first reflected light beam toward the first digital imager and to reflect at least a portion of the second reflected light beam toward the second digital imager; wherein the first digital imager is configured to generate a first digital image based at least on the first reflected light beam and wherein the second digital imager is configured to generate a second digital image based at least on the second reflected light beam, and wherein the processor system is configured to determine a distance between the reference plate and the raised surface feature of the sample based at least on the first and second digital images.
 10. The optical system of claim 9, wherein the first reflected light comprises a coherent addition of reflections of the first light beam on the reference plate, on the raised surface feature and on the sample surface and wherein the second reflected light comprises a coherent addition of reflections of the second light beam on the reference plate, on the raised surface feature and on the sample surface.
 11. The optical system of claim 9, wherein the processor system is further configured to generate an image of the sample surface for defect inspection and to generate a fluorescence image of the sample for residue-defect inspection.
 12. The optical system of claim 9, wherein the second beam splitter is a dichroic mirror configured to primarily transmit a first pre-determined band of wavelengths and to primarily reflect a second pre-determined band of wavelengths, wherein the first light beam has a wavelength within the first pre-determined band of wavelengths and wherein the second light beam has a wavelength within the second pre-determined band of wavelengths.
 13. The optical system of claim 9, wherein the first digital imager and the second digital imager are configured to generate digital images based on sample fluorescence emission, generated via excitation of the sample surface and of the raised surface feature by the first and second light beams.
 14. The optical system of claim 9, wherein the reference plate positioned above the surface of the sample has an optical thickness configured to provide a near common path Mirau interferometer functionality.
 15. The optical system of claim 9, further configured to optically scan the sample surface using a translation system whose travel speed across a field of view of the optical system determines a sampling pixel size of the first and second images.
 16. The optical system of claim 15, wherein the translation system is configured to at least one of move the sample and move the optical system.
 17. The optical system of claim 9, wherein the first light beam has a first wavelength and is emitted at a first time, and wherein the second light beam has a second wavelength that is different form the first wavelength and is emitted at a second time that is different than the first time, and wherein the processor system is configured to determine the distance between the reference plate and the raised surface feature based on the first digital image and the second digital image.
 18. The optical system of claim 9, wherein the tunable light source is configured to emit a plurality of light beams, the plurality of light beams including the first light beam and the second light beam, each of the plurality of light beams having a different wavelength, a number of the plurality of light beams being selected based on the distance between the reference plate and the raised surface feature, and wherein the determined distance between the reference plate and the raised surface feature being based on a plurality of first and second images generated by the first and second digital imagers based on the plurality of light beams.
 19. The optical system of claim 9, wherein the tunable light source comprises: a broadband light source configured to emit the first light beam at a first time and the second light beam at a second time; and a tunable filter, wherein the tunable filter is configured to filter the first light beam to have a first wavelength and to filter the second light beam to have a second wavelength.
 20. The optical system of claim 9, wherein the distance between the reference plate and the raised surface feature is determined based on a first intensity value of a first pixel location in the first digital image and a second intensity value of a corresponding first pixel location in the second digital image.
 21. The optical system of claim 9, wherein the first lens system is positioned between the sample and the first beam splitter and wherein the optical system further comprises: a second lens system positioned between the first and second beam splitters; and an adjustable system aperture positioned one of: in an aperture plane of the first lens system, and between the first lens system and the second lens system.
 22. The optical system of claim 21, wherein a size of an aperture of the adjustable system aperture is configured to be adjustable based on at least one of a lateral resolution and a field of view of the optical system.
 23. The optical system of claim 9, wherein the first beam splitter is controllable to redirect the first light beam away from the raised surface feature toward another part of the sample, such that the raised surface feature is unilluminated by the redirected first light beam; wherein the processor system is configured to determine a distance between the reference plate and the other part of the sample.
 24. The optical system of claim 9, wherein the second digital imager is configured to receive at least a portion of the first light beam that includes a reflected light beam from normal incidence illumination and at least a portion of the second light beam that includes scattered light beam from off-axis illumination to generate at least one third digital image based on the reflected and scattered light beams and wherein the processor system is further configured to generate a two-dimensional digital image from the at least one third digital image.
 25. The optical system of claim 9, wherein the sample is moved under the imaging system by a translation system comprising a moving stage and wherein the second digital imager is configured to receive at least a portion of the first light beam that includes a reflected light beam from normal incidence and at least a portion of the second light beam that includes scattered light beam from off-axis illumination to generate at least one third digital image based on the reflected and scattered light beams and wherein the processor system is further configured to generate a two-dimensional digital image of an entire surface of the sample from the at least one third digital image.
 26. The optical system of claim 9, wherein the tunable light source is configured to emit a single wavelength of light beam toward the sample and to enable repeated multiple single wavelength inspection of the sample in both a bright field mode and in a dark field mode.
 27. The optical system of claim 26, wherein the single wavelength of light beam incident on the surface of the sample causes at least a portion of the surface of the sample to fluoresce, wherein the second digital imager is configured to receive a fluorescence emission from the sample surface via a first dichroic mirror as the sample is moved under the imaging system by translation system comprising a moving stage; and wherein the second processor system is configured to generate a two-dimensional digital image based on the received fluorescence emission.
 28. The optical system of claim 9, further comprising an auto-correlation interferometer comprising a first movable mirror and a second movable mirror both disposed along an optical path of the second reflected light beam and aligned with the second digital imager.
 29. The optical system of claim 9, wherein the first light beam comprises a wavelength or a band of wavelengths that causes the fluorescence emission at the sample.
 30. The optical system of claim 9, wherein at least one of the first and second digital imagers is configured to receive at least a portion of the fluorescence emission and to generate a fluorescence image. 